1. Field of the Invention
The invention relates to a pneumotachograph mask or mouthpiece coupling element for airflow measurement, and more particularly, to a pneumotachograph mask or mouthpiece coupling element for use with a pneumotachograph during speech or singing.
2. Description of the Related Technology
Measurements of the volume-velocity of the airflow exiting the mouth or the nose during speech or singing are useful in evaluating the efficiency of voice production and measuring the pattern of valving of the airflow stream by the articulators. Systems for measuring this airflow, often referred to as pneumotachographs, can be categorized by their frequency response.
The acoustic or audible voice frequencies begin at about 50 Hz. (The threshold of audibility is somewhat lower 15 to 20 Hz.) When measuring only the average airflow in each of a sequence of speech sounds (and not detailed variations within each speech sound), the upper frequency limit is determined by how fast the articulatory organs can produce the sounds. This limit is about 10 Hz to 15 Hz, as witnessed by the way children count seconds using the sequence "One Mississippi, Two Mississippi, etc." (Mississippi has eight speech sounds.) When measuring airflow in an artificially prolonged vowel sound (as "ahhh"), a system frequency response limitation even lower, say 2 or 3 Hz, is sufficient.
Airflow measurement systems restricted to frequencies below about 20 Hz may determine the rate of lung deflation, the overall aerodynamic efficiency of the voice source, and may measure articulatory valving patterns, as during articulation of consonants in which there are sudden changes in flow or an onset or offset of voice.
On the other hand, to measure the details of the individual repetitive pulses of airflow making up the voice, a good frequency response to at least about 1000 Hz is required. This frequency response is needed to accurately capture the voice fundamental frequency component, which is generally in the range of 100 to 300 Hz, and at least 3 or 4 harmonics, in order to adequately define the pulse wave shape.
For the purpose of describing the invention, one need only differentiate between systems measuring low, principally subsonic, frequencies (below about 20 Hz) and systems which also measure the higher, acoustic frequencies (well above 20 Hz).
Early devices used to measure the low frequency airflow, as well as some current commercial devices, are adaptations of methods used for measuring non-speech respiratory airflow in breathing. In these systems, a solid-walled pneumotachograph mask with an airtight seal to the face is held over the subject's mouth, nose or the combined mouth and nose.
Alternatively, the subject breathes through a mouthpiece having an airtight seal to the lips while the nose is held closed.
The mask or mouthpiece funnels the air stream through a volume-flow measurement transducer, such as an intervening tube of some convenient length that need only be much less than a wavelength at the highest frequency to be measured. (A wavelength at 20 Hz is roughly 50 feet.)
Such systems have an inherently limited ability to measure the energy at higher, acoustic frequencies, due to the resonances and other acoustical properties of the mask or tube used to funnel the airflow. These measurement limitations are present even if the flow transducing mechanism is capable of tracking fast variations. This frequency response limit constrains the system from measuring the faster variations of airflow, which occur during the release of a stop consonant or during the individual airflow pulses produced by the voice source.
These methods, i.e., funneling methods, may also have a number of disadvantages, when measuring low frequency airflow. Such disadvantages are present with masks or mouthpieces constructed according to the prior art, when the system is used during speech or singing, as opposed to breathing. The invention is directed toward solving the following problems:
(1) The mask and tube become, in effect, part of the acoustical system formed by the mouth and pharynx and thus alter the vowel or consonant being formed. The resulting change in vocal tract acoustics can include a change in the airflow variables being measured and a change in the quality of the sound produced. For brevity, these changes in the nature of the voice may be referred to as voice distortion.
(2) The mask and tube block or muffle the voice wave transmitted to the surrounding air, so the speaker or a clinician making the measurements may have a greatly distorted perception of the loudness and quality of the voice being produced. This effect may be referred to as voice muffling.
(3) Transducers that measure only unidirectional flow, such as hotwire anemometers and certain mass-flow transducers, cannot be directly employed, even when measuring only the slowly-varying or average egressive airflow during speech or singing. Unidirectional flow measuring transducers are disadvantageous with funneling methods because the voice, unlike the airflow in normal breathing, contains strong high frequency oscillations. When the high frequency oscillations of the voice are stronger than the average airflow, momentary reversals in the instantaneous airflow may result. Thus, the flow transducer in a conventional funneling measurement system must be able to transduce these flow reversals (ac flow components) even if they are to be eliminated in the final system output signal by some form of low-pass filtering or time-averaging.
One method for eliminating or greatly ameliorating the above problems caused by a funneling mask or mouthpiece is disclosed in "A new inverse-filtering technique for deriving the glottal airflow waveform during voicing," J. Acoust. Soc. Amer., Vol. 53, No. 6, pp. 1632-1645, 1973 by M. Rothenberg, the disclosure of which is expressly incorporated herein. According to this method, a "circumferentially-vented" mask or mouthpiece may be utilized.
The mask or mouthpiece, according to the Rothenberg article, includes a flow resistance element, such as a fine-mesh wire screen, which is used as to convert the flow variable to an air pressure variable. The wire screen may cover openings that are distributed over the surface of either a face mask or a mouthpiece, and are located as close to the mouth as feasible. The wire screen may be combined with a differential pressure transducer that measures the small pressure difference across the screen caused by the airflow.
The circumferentially-vented mask may be used to measure variations in airflow that occur in less than a millisecond (well into the acoustic range) and may cause relatively little distortion and muffling of the voice. A window area of about 10 cm.sup.2 in a mask chamber covering the mouth and nose should have an impedance of less than about 0.5 cm H.sub.2 O/liter per sec, in order to be considered a low impedance acoustic barrier in a typical speech application. More sensitive applications, such as in measurements during singing, might require a window impedance of no more than 1/2 or 1/4 that value. Conversely, larger window areas can have a somewhat higher overall impedance for the same muffling effect.
However, these types of masks cannot be conveniently applied to transduction methods other than the resistance-pressure method. For example, mass-flow transducers that require a unidirectional flow cannot be used. Further, the presence of a flow resistance element so close to the mouth makes the mask or mouthpiece susceptible to contamination that can change calibration.
For applications that record only the slowly-varying components of the voice airflow, the short response time (or, equivalently, the extended high frequency response) of a circumferentially-vented mask is not a factor. For these low frequency applications there exists a need for a measurement system that does not significantly distort or muffle the voice.
It is also desirable to locate a transduction mechanism at a greater distance from the mouth than the distance typically accommodated by a wire-screen of a circumferentially-vented mask or mouthpiece. There also exists a need for a low frequency system useable with transducers which require a unidirectional flow for proper operation.